Video is generally represented as sequences of frames in accordance with either the interlaced or the progressively-scanned format (non-interlaced). Each frame includes a matrix of pixels that vary in color and intensity according to the image displayed.
Referring to FIG. 1, in the interlaced scan format, a frame, which is a raster array of image bytes representing an image, includes a pair of fields, in which a field is a raster array of image bytes representing every other row of a frame and are derived from two different instants. The primary field of the pair of fields, for example, is the input field associated with the time instant for which the output frame is to be constructed and includes pixels that are located only on alternate rows (either odd or even rows) of the frame matrix, called horizontal lines. The secondary field includes pixels that are located on the corresponding horizontal lines of the frame matrix which are the missing pixels in the primary field. The pixels in the secondary field represent the portions of the image not represented in the primary field. The primary and secondary fields of a frame are scanned consecutively, for example, on a video display monitor at a rate of 60 fields/sec for purposes of reconstructing the entire image on the display at the industry interlaced scan standard 30 frames/sec display rate.
In the progressively scanned format, an image is represented in its entirety using only a single field that includes pixels in all horizontal lines of the frame matrix. Therefore, such frames can be progressively scanned on a display at the standardized progressive display rate of 60 frames/sec.
Conventional television systems receive frames of video signals in an interlaced format. For example, the National Television System Committee (NTSC) standard is used to send and receive frames of television signals at a rate of 30 frames/second. Each frame contains 525 lines of video scan lines, which are divided into two interlaced fields. The interlaced fields are transmitted at a rate of 60 fields/second, or 30 frames/second. The receiver scans the two interlace fields of each frame, one by one, to display the interlaced video signals as television pictures.
Several video applications also use interlace scanning during image origination or capture as well as during signal transmission from the encoder, which codes the video signal, to the receiver. For example, digital video compression methods, such as the ISO (International Standards Organization) MPEG (Moving Pictures Expert Group) video compression standard, may be used to reduce the data rate to a level suitable for transmission over an available digital channel.
However, the display for these digitally compressed video image sequences at the decoders may not use interlaced scanning, or it may be desirable to use non-interlaced displays. For example, in large screen television displays, multimedia displays, or computer displays that support many text-oriented or graphics-oriented applications, a non-interlaced format is often preferred over an interlaced format for a variety of reasons, such as the lower costs associated with implementing the progressively scanned format technology.
Thus, there is a need to convert an interlaced format to a non-interlaced format. The process of converting an interlaced format to a non-interlaced format is generally referred to as deinterlacing (or line-doubling). Referring to FIG. 1, each pair of pixels, e.g. pixels 101 and 102, contains four 8-bit values. The four values include two luminance values, one Red-chroma (Cr) value and one Blue-chroma (Cb) value. Deinterlacing involves comparing these four values in the primary field with their respective values in the secondary field and deinterlacing the values independently of each other. Deinterlacing can be used to convert interlaced still pictures to non-interlaced still pictures, or to provide display of interlaced video on a progressive display (non-interlaced) computer monitor.
Several techniques for deinterlacing a sequence of frames in the interlaced scan format have been developed to provide images to be displayed on the higher image quality, progressively scanned format display equipment. A simple technique is to merge the odd and even fields. However, this technique causes spatial artifacts. Other conventional deinterlacing techniques utilize fixed numerical thresholds to adapt to changing image content between fields (i.e., to minimize spatial artifacts) or utilize computationally intensive approaches, such as contour deinterlacing or spatio-temporal deinterlacing interpolation for estimating the values of missing pixels in an interlaced frame.